Conventional modulation and demodulation methods for ultra wide band impulse communication are as follows:                Pulse Position Modulation/Bi-orthogonal Pulse Position Modulation (PPM/BPPM)        Pulse Amplitude Modulation (PAM) including On-Off Keying method        Phase Shift Keying (PSK) including Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK)        Direct Sequence Code (DSC)        
First, the PPM/BPPM, a method used and studied for a long time, changes the position of a basic pulse according to 0 or 1. According to the PPM/BPPM where modulation is performed through temporal offset, a signal arriving earlier than a reference time can be determined as ‘0’ and a signal after the arrival of the signal 0 is determined as ‘1.’ A method using an M-ary orthogonal signal can be applied to the PPM ultra wide band modulation/demodulation method to improve a transmission rate and a signal-to-noise ratio. The BPPM method uses one-digit binary data for pulse phase modulation when the M-ary method is used. In case where a binary data signal is transmitted, the BPPM method becomes the same as the BPSK modulation method. As it can be seen from the above description, it is possible to transmit data twice as much as the conventional PPM and to extend a time period occupied by one pulse because a binary datum is transmitted on a phase basis.
Secondly, the PAM method displays information by changing an output. In 8-ary PAM, three-digit binary data are transmitted by using amplitudes in 8 steps. A special form of the PAM is the OOK method in which ‘1’ is transmitted when there is a pulse and ‘0’ when there is no pulse. The BPSK method displays ‘0’ and ‘1’ according to the phase of a signal. The BPSK is advantages over the PPM in the signal-to-noise ratio by 3 dB; in a transmission capacity by two-folds; and in the time period occupied by a pulse by two-folds. However, in the presence of multipath, there is a problem that the phase is changed. The QPSK is a form extended from the BPSK. Whereas the BPSK identifies a signal with ‘0’ or ‘1,’ the QPSK identifies a signal with two-digit binary data, e.g., ‘00,’ ‘01,’ ‘10’ and ‘11.’ Since the QPSK method transmits two-digit binary data with one waveform, it can transmit data twice as much as the BPSK can.
The DSC method which adopts the Direct Sequence-Code Division Multiple Access (DS-CDMA) without any change has advantages that it can prevent the generation of noise excellently and data from being intercepted in the course.
However, the above-described conventional modulation technologies for realizing ultra wide band impulse communication has shortcoming that they can hardly applied to multipath channels and that they have a limitation in data transmission rates.